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Your question has nothing to do with Zener diodes, but anyway... The simplest way to dim a string of LED's is by adding a series resistor. To determine the value of that resistor we would need to have the full specs on the LED. But based on the "10W" rating, try a 3.3 ohm 5 watt resistor.

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PWM basically uses a fixed freqency but variable duty cycle to control power. Imagine turning a heater on and keeping it on - that is 100% duty. Now turn it on for 10 seconds and off for 10 seconds - that is 50% duty cycle. Now on for 2 seconds and off for 18 seconds - 10% duty cycle.

You can do the same for bulbs and LEDs - you need a much faster frequency though - 200Hz or upwards or you'll see the flickering.

If you were to use a voltage / current dropping device, the power you want to be removed from the LEDs has to be burned up as heat - this is inefficient and can be a pain to get rid of.

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Your question has nothing to do with Zener diodes, but anyway... The simplest way to dim a string of LED's is by adding a series resistor. To determine the value of that resistor we would need to have the full specs on the LED. But based on the "10W" rating, try a 3.3 ohm 5 watt resistor.

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I did use a formula and it came up as 4.0 ohm 5 watt. Damn I was close.
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The power is supplied thru relays. Is there a need to use a diode to limit feed back ? Or just a precaution.

Both power input will have power at the same time. I guess I need to cancel/block. They need to be independent application. to Leds

When the leds are on, they are consent 10v dimmed. They are only brighter full 12v when brake circuit is applied.

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10W at 12V is about 0.83A, it may be slightly higher as cars tend to run at 13.8V-14.8V.

Assume though 10W @ 12v so you'll be dropping 2v @ 0.83A which is 1.66W - it will get hot to the touch when running so I'd personally look at a small metal cased resistor which you could screw on to the chassis to act as a very large heatsink !

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It is illegal to tamper with vehicle lighting in my country because many beginners make the lights too dim, too bright or the wrong color.
The drawing called, "Something like this" might feed 14.4V from the alternator through the diode producing 13.7V. Since you did not post details about the LEDs then they might have an absolute maximum voltage of 9V (to 12V) and blow up. LEDs are usually fed a limited current, not a voltage.

Warm White brake lights? Or the white Leds are behind a red lens?
But a white LED is actually a blue LED and a yellow phosphor which produces not much red light. That is why car makers use red LEDs for brake lights and rear running lights. They use PWM for dimming.

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It is illegal to tamper with vehicle lighting in my country because many beginners make the lights too dim, too bright or the wrong color.
The drawing called, "Something like this" might feed 14.4V from the alternator through the diode producing 13.7V. Since you did not post details about the LEDs then they might have an absolute maximum voltage of 9V (to 12V) and blow up. LEDs are usually fed a limited current, not a voltage.

Warm White brake lights? Or the white Leds are behind a red lens?
But a white LED is actually a blue LED and a yellow phosphor which produces not much red light. That is why car makers use red LEDs for brake lights and rear running lights. They use PWM for dimming.[/QU
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Yes I did post the Specs for the Leds in the question.... 10W 900LM High Bright LED Light Lamp Chip DC 9-12V

I am from the Automotive Proto- type side of the auto industry. quite aware of the US reg.

The mention of diodes was to get attention!

The pic of the tail light, I had room for independent rows of Led's. Thus not a problem high's / low's